The present invention relates to a circuit for detecting a level of an input voltage, and particularly to a level detection circuit composed of insulated gate field effect transistors.
A level detection circuit employing a ratio circuit and an inverter circuit has been proposed to detect the level of an input voltage, particularly to detect that the input voltage is larger in absolute value than a power supply voltage. The ratio circuit includes first and second insulated gate field effect transistors (IGFET's) connected in series between power supply terminals. The first IGFET is of an enhancement type and supplied with the input voltage, and the second IGFET is of a depletion type. The connection point of the first and second IGFET's is an output terminal of the ratio circuit to which the inverter circuit is connected.
The output level of the ratio circuit is changed in accordance with the ratio of the internal impedances of the first and second IGFET's and the internal impedance of the first IGFET is controlled by the input voltage. When the output level of the ratio circuit exceeds the threshold level of the inverter circuit, the inverter circuit changes its output logic level.
Thus, the detection level of the input voltage is determined by the internal impedances of the first and second IGFET's and the threshold level of the inverter circuit. Accordingly, various detection levels of the input voltage can be designed with facility, and thus it is possible to generate the detection output when the input voltage exceeds the power supply voltage.
However, precise control of the threshold level of the inverter circuit is very difficult. Moreover, the internal impedances of the IGFET's depend on the threshold levels thereof, and it is also quite difficult to precisely determine the threshold levels of the IGFET's. As a result, the detection level of the input voltage deviates considerably from a designed value.